AS PERCEIVED BY THE EYE. 277 



how to interpret their answers. The observer should not look at the coloured 

 papers, nor be told the proportions of the colours during the experiments. When 

 these adjustments have been properly made, the resultant tints of the outer and 

 inner circles ought to be perfectly indistinguishable, when the top has a sufficient 

 velocity of rotation. The number of divisions occupied by the different colours 

 must then be read off on the edge of the plate, and registered in the form of an 

 equation. Thus, in the preceding experiment we have vermilion, ultramarine, 

 and emerald green outside, and black and white inside. The numbers, as given 

 by an experiment on the 6th March 1855, in daylight without sun, are — 



•37 V + -27U+-36EG=-28 SW + -72Bk (1). 



The method of treating these equations will be given when we come to the theo- 

 retical view of the subject. 



In this way we have formed a neutral gray by the combination of the three 

 standard colours. We may also form neutral grays of different intensities by the 

 combination of vermilion and ultramarine with the other greens, and thus obtain 

 the quantities of each necessary to neutralize a given quantity of the proposed 

 green. By substituting for each standard colour in succession one of the colours 

 which stand under it, we may obtain equations, each of which contains two stand- 

 ard colours, and one of the remaining colours. 



Thus, in the case of pale chrome, we have, from the same set of experiments, 



•34PC + -55U + -12EG = -37 SW + -63Bk . . . . (2). 



We may also make experiments in which the resulting tint is not a neutral 

 gray, but a decided colour. Thus we may combine ultramarine, pale chrome, and 

 black, so as to produce a tint identical with that of a compound of vermilion and 

 emerald green. Experiments of this sort are more difficult, both from the inabi- 

 lity of the observer to express the difference which he detects in two tints which 

 have, perhaps, the same hue and intensity, but differ in purity ; and also from 

 the complementary colours which are produced in the eye after gazing too long at 

 the colours to be compared. 



The best method of arriving at a result in the case before us, is to render the 

 hue of the red and green combination something like that of the yellow, to reduce 

 the purity of the yellow by the admixture of blue, and to diminish its intensity by 

 the addition of black. These operations must be repeated and adjusted, till the 

 two tints are not merely varieties of the same colour, but absolutely the same. 

 An experiment made 5th March gives — 



•39PC + -21U + -40Bk = -59 V + -41EG (3). 



That these experiments are really evidence relating to the constitution of the eye, 

 and not mere comparisons of two things which are in themselves identical, may 

 be shown by observing these resultant tints through coloured glasses, or by using 



